1. Field of the Invention
The present invention relates to an automatic frequency sweeping method for controlling a frequency synthesizing type autotuners for broadcasting receivers, and more specifically to an intermediate frequency counting method for stopping the automatic frequency sweeping in the above mentioned method.
2. Description of Related Art
In general, broadcasting receivers incorporating therein frequency synthesizing type autotuners are so adapted to able to detect whether or not a broadcasting signal exists by counting an intermediate frequency. For example, in so-called FM band, assuming that a local oscillator is a lower side oscillator of the phase locked loop (PLL) type and a broadcasting signal exists at a frequency of 80MHz, if the lower side local oscillator generates an signal of 69.3 MHz, an intermediate frequency of 10.7MHz is obtained as a well known manner, and a result, it is possible to receive the broadcasting frequency signal of 80 MHz. At this time, if the oscillation frequency is deviated with 100 KHz, the intermediate frequency is also deviated with 100 KHz from 10.7 MHz. Therefore, taking into consideration the degree of modulation and noises of the broadcasting frequency, it is allowed to judge that a broadcasting frequency signal exists if the intermediate frequency is in the range of 10.7 MHz.+-.25 KHz.
Thus, in broadcasting receivers adapted to execute the automatic frequency sweeping in response to a key input, the automatic frequency sweeping operation is performed as follows: When the key input is given, the oscillation frequency is shifted by a frequency corresponding to one channel space by action of the PLL and associated controller. Then, after the PLL is locked, presence of the intermediate frequency is discriminated. If no broadcasting signal is detected, the oscillation frequency is shifted by the channel space again and presence of the intermediate frequency is discriminated. This operation is repeated, and when the broadcasting signal is judged to exist, the automatic frequency sweeping is stopped.
For example, in the FM band of Japan, frequencies are allocated to FM broadcasting stations with frequency intervals of 100 KHz, and in the FM band of U.S.A., frequencies are allocated to FM broadcasting stations with frequency intervals of 200 KHz. Further, in Europe, frequencies are, as a general rule, allocated to FM broadcasting stations with frequency intervals of 100 KHz. Therefore, if a receiving frequency is changed (incremented or decremented) in accordance with the automatic frequency sweeping, the frequency of the local oscillator has to be changed with only an inter-station frequency (one channel space), and then, after the PLL is locked, a gate is opened for only 4 milliseconds so that the intermediate frequency signal is supplied to a so-called intermediate frequency counter. Thus, the intermediate frequency counter counts the intermediate frequency signal for a period of 4 milliseconds. Here, assuming that when the intermediate frequency is in the range of 10.7 MHz.+-.25 KHz as mentioned above, it is judged that the broadcasting signal is present, when the value of intermediate frequency counter is in the range of 42,800.+-.100, the automatic frequency sweeping is stopped. If the value of the intermediate frequency counter is not in the above mentioned range, the frequency of the local oscillator is changed with 100 KHz again, and the same operation is repeated.
If the range for judging the presence of the broadcasting frequency is changed, it is possible to perform the automatic frequency sweeping in the AM band in the same manner.
However, in fact, if the presence of the broadcasting signal is judged on the basis of only one counting of the intermediate frequency for 4 milliseconds, possibility of mis-judgment cannot be denied. Therefore, the counting operation of 4 milliseconds is repeated three times and if at least two of three counting operations are in the range mentioned above, it is then judged that the broadcasting signal is present.
As seen from the above, the conventional automatic frequency sweeping has required at least 12 milliseconds for counting the intermediate frequency. Therefore, the sweeping from a minimum frequency channel to a maximum frequency channel will take a very long time, particularly in a band such as FM band including a number of channels.